1. Field of the Invention
The present invention relates to magnetic sensing elements which utilizes a tunneling effect and which is to be mounted on magnetic reproducing devices, such as hard disk drives, or other magnetic sensing devices. More particularly, the invention relates to a tunneling magnetic sensing element which includes a free magnetic layer having low magnetostriction λ and which has a high rate of change in resistance (ΔR/R), the element thus having excellent magnetic sensitivity and stability, and a method for manufacturing the same.
2. Description of the Related Art
A tunneling magnetic sensing element (tunneling magnetoresistive element) causes a change in resistance utilizing a tunneling effect, wherein when the magnetization direction of a pinned magnetic layer is antiparallel to the magnetization direction of a free magnetic layer, a tunnel current does not easily flow through an insulating barrier layer (tunnel barrier layer) provided between the pinned magnetic layer and the free magnetic layer, and the resistance is at a maximum. On the other hand, when the magnetization direction of the pinned magnetic layer is parallel to the magnetization direction of the free magnetic layer, the tunnel current flows easily, and the resistance is at a minimum.
By use of the principle described above, a change in electrical resistance caused by a change in the magnetization of the free magnetic layer under an influence of an external magnetic field is captured as a change in voltage to detect a leakage magnetic field from a recording medium.
The following Patent Documents 1 to 5 disclose a tunneling magnetic sensing element having a protective layer with a single-layered or two-layered structure containing Ti or Ru or a method for manufacturing the tunneling magnetic sensing element.
Patent Document 1:
Japanese Unexamined Patent Application Publication No 2002-094142
Patent Document 2:
Japanese Unexamined Patent Application Publication No 2002-374020
Patent Document 3:
Japanese Unexamined Patent Application Publication No 2006-190780
Patent Document 4:
Japanese Unexamined Patent Application Publication No 2000-228003
Patent Document 5:
Japanese Unexamined Patent Application Publication No 2005-032780
In tunneling magnetic sensing elements, problems to be solved include increasing detection sensitivity by achieving a high rate of change in resistance (ΔR/R) to improve the characteristics of a read head, and suppressing noise in read heads by reducing the magnetostriction λ of a free magnetic layer to a value close to zero to enhance stability.
In existing tunneling magnetic sensing elements, a protective layer composed of tantalum (Ta) is disposed on the free magnetic layer.
However, when heat treatment is performed, Ta diffuses into the free magnetic layer or induces interfacial strain and interfacial stress with respect to the free magnetic layer, resulting in an increase in the magnetostriction λ of the free magnetic layer.
For example, when an insulating barrier layer is composed of magnesium oxide (Mg—O), it is known that, in order to increase the rate of change in resistance (ΔR/R) of the tunneling magnetic sensing element, it is preferable to provide an enhancement layer having a body-centered cubic (bcc) structure in a portion of the free magnetic layer in contact with the insulating barrier layer. However, when the enhancement layer is provided, the magnetostriction of the free magnetic layer increases. Therefore, a material that decreases the magnetostriction of the free magnetic layer is used for the portion other than the enhancement layer of the free magnetic layer so as to prevent the magnetostriction of the free magnetic layer from increasing.
However, even when such a structure in which the rate of change in resistance (ΔR/R) can be increased and the magnetostriction λ of the free magnetic layer can be decreased is employed, if a protective layer composed of Ta is disposed on the free magnetic layer, as above, the magnetostriction λ of the free magnetic layer eventually increases as the heat treatment causes Ta to diffuse into the free magnetic layer. Moreover, diffusion of Ta into the enhancement layer or insulating barrier layer induces strain in the (bcc) structure and thus inhibits crystallization, thereby decreasing the rate of change in resistance (ΔR/R).
When the protective layer is composed of Ta, moreover, since a milling rate of Ta is lower than those of materials used for forming other layers such as the free magnetic layer and the insulating barrier layer, the protective layer projects from the vicinity of the insulating barrier material after milling of a surface for facing a recording medium (Air Bearing Surface, hereinafter referred to as “ABS”). This causes a spacing loss with the recording medium, resulting in problems of a decrease in sensitivity and deterioration of SN ratio. Consequently it has been desired that the ABS can be made as flat as possible to prevent projection of the protective layer by composing the protective layer of a material having almost the same milling rate as those of materials used for forming other layers.
The above-identified Patent Documents 1 to 3 disclose a tunneling magnetic sensing element, in which a protective layer or a mutual diffusion layer including Ti is disposed on a free magnetic layer, or a method for manufacturing the tunneling magnetic sensing element. In any case, a single layer of Ti is disposed on the free magnetic layer.
The above-identified Patent Document 4 discloses a tunneling magnetic sensing element, in which a protective layer is a single-layeres film composed of a metal such as Ru or a two-layered film composed of TiW/Ru or the like. However, in such a two-layered film Ta or TiW layer is disposed directly on a free magnetic layer, and therefore it fails to disclose a two-layered protective layer having a platinum-group element directly on the free magnetic layer and having Ti on the platinum-group element.
The above-identified Patent Document 5 discloses a tunneling magnetic sensing element, in which a protective layer has a two-layered structure of Ti/Ru or the like. However, the metal layer to be disposed on a free magnetic layer is composed of Ti, Ta or the like, and therefore it fails to disclose having a platinum-group element on the free magnetic layer and having Ti on the platinum-group element. In addition, since Ta is used for a two-layered film disclosed in its embodiments, heat treatment causes diffusion of Ta to thereby increase the magnetostriction λ of the free magnetic layer, as described above.
If the protective layer does not contain Ta, the problems of an increase in the magnetostriction λ and a decrease in the rate of change in resistance (ΔR/R) will never be caused by diffusion of Ta into the free magnetic layer or the like. This also prevents the protective layer from projecting from the ABS after milling.
In the above-identified Patent Documents 1 to 5, however, since a single layer of Ti is disposed on the free magnetic layer as the protective layer or in the case where the protective layer has a multilayered structure, the layer to be in contact with the free magnetic layer is composed of Ti, Ti diffuses into the free magnetic layer and the insulating barrier layer to cause the problem of an increase in the magnetostriction λ.